1. Field of the Invention
This invention relates to a pile, that is, a so-called gravel drain pile comprising aggregates such as crushed stones, slag, gravel or cobbles, etc. having permeability and bearing capacity placed on a relatively loose sand foundation saturated with ground water, and more specifically to a method for automatically driving a gravel drain pile and an execution apparatus for carrying out the method which compacts the foundation in the periphery of the piles during the process of driving the gravel drain piles.
2. Description of the Prior Art
The present applicants have previously proposed the invention of a method for driving a gravel drain pile of this kind and an execution apparatus therefor in Japanese Patent No. 1,432,555 (Patent Publication No. 62(1987)-40,482, hereinafter referred to as "prior art").
That is, the method for driving a gravel drain pile according to the aforesaid invention is characterized by interpenetrating a hollow casing, an extreme end of which is closed, into predetermined depth, thereafter charging aggregates for driving a gravel drain pile into the casing and releasing the aggregates out of the extreme end of the casing, placing a compaction rod at a compacting position adjusted to the soil of the peripheral foundation and the grain size of aggregates, said compaction rod being disposed within the casing, transmitting impact force to the charged aggregates to compact the aggregates, and continuously performing the charging of the aggregates and the compacting with the compaction rod.
The driving apparatus according to the aforesaid invention comprises a hollow casing having an open- and closable lid at the extreme end thereof; a compaction rod which has a substantially equi-section, has a small diameter and is lengthy, said compaction rod being inserted from the lower end to the upper portion of the casing along a center axis within the casing; an impact drive device for a compaction rod disposed upwardly of the casing and being operatively connected to the compaction rod to transmit impact force to the compaction rod; and rod-height adjusting device for variably adjusting movement of an extreme end surface of the compaction rod.
That is, the invention according to prior art has aimed at compacting action by the compaction rod on crushed stones to achieve driving an effective gravel drain pile.
The aforesaid prior art has already proposed (1) the compaction of the peripheral ground by tamping during the driving process of the gravel drain pile can be expected, and (2) the gravel drain pile is driven while controlling decision factors of a tamping degree such as a raising speed of a hollow pipe, i.e., a casing, a period and amplitude of the compaction rod and a height of the extreme end surface thereof or a charging amount of crushed stones in accordance with the tamping degree determined while adjusting to the peripheral soil and the grain size of aggregates.
However, in the existing circumstances, it is not easy to control these factors, and such control greatly depends upon operators' experiences or intuitions. Therefore, it is also difficult to compact the peripheral foundation to the degree as desired.
The strength of the compacted ground is merely judged by a sounding test after a gravel drain pile has been driven. Even though the peripheral ground around the pile was not compacted enough to get higher strength, the pile can not be re-driven and it is left as it is. Therefore, the former method has a problem on quality control of ground compaction and becomes the bottleneck in raising efficiency of pile driving.